Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. The transmission speed ratio is the ratio of input shaft speed to output shaft speed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising.
Most transmission are equipped with a torque converter or other type of launch device. When the vehicle is stationary or moving very slowly, the gearbox input speed is less than the minimum operating speed of the engine. A launch device transmits torque from the engine to the gearbox input while permitting the engine to rotate at an acceptable speed. A torque converter includes an impeller driven by the engine and a turbine driving the gearbox input. Torque is transferred from the impeller to the turbine hydro-dynamically.